Traditional peripheral lead integrated circuit (IC) test sockets are generally constructed of a machined engineering grade plastic housing and frame, which is populated with a series of contact members that correspond to the terminals on two or four sides of the IC device. These contacts provide a temporary connection between the IC device terminals and the appropriate location on a test printed circuit board (PCB), also called a test or load board. This temporary electrical connection provides the mechanism to power the IC device and test performance.
The IC devices are graded based upon performance, and the devices that pass are identified while the devices that fail are either retested or discarded. In many cases, the performance windows in which the IC devices must operate can be fairly precise. Accordingly, the electrical performance of the contact members in the socket are important so as to not introduce degradation in signal or power delivery. A socket with good electrical performance is critical for many IC testing functions since poor performance can result in retesting or discarding devices that are actually acceptable but failed the test due to degradation within the socket.
IC manufacturers and their sub-contractors test millions or even billions of devices, and it is important for the socket to not only perform well electrically, but to also be mechanically robust enough to last hundreds of thousands of test cycles before being replaced. Many of the traditional sockets utilize very small metal contacts that are biased using elastomers. These contact members are mechanically compressed as the device is pushed against the contacts. An elastomeric material functions like a spring and provides the normal force to return the contact to the original position. The elastomeric material functioning as a spring member allows the contact members to be much shorter than they otherwise would have to be in order to fashion a spring in conjunction with a contact member. The short length provides an electrical path that has fewer distortion effects as the current or signal passes through the contact. In some cases, spring probes are used which contain opposing metal contacts that are biased by a coil spring.
There are a wide range of contact types used in traditional sockets. In general, they all have a range of mechanical life that is limited by the effects of contacting the solder that is plated onto the terminals of the IC device. As the solder collects on the tips of the contacts, the Tin within the solder oxidizes to form a high resistance layer that causes the contact to lose electrical performance.
In some cases, the contacts move with respect to the interface to the circuit board as the device actuates the contact. This movement can cause wear on the PCB which can degrade performance of the circuit board. In general, the need for thousands of cycles before replacement drives sophisticated design that can be expensive. The socket volumes can also be relatively small requiring custom production methods that can also be expensive. The methods used to produce the plastic socket housings which hold the contacts are typically expensive precision machining operations. Many of the sockets are assembled by hand, where the elastomeric members and contacts are inserted by hand under magnification.
Socket users typically look for the best mix of electrical performance, yield, cost, and mechanical life to determine the costs to test each IC device. Electrical performance can be a hurdle that eliminates many of the socket options. Those sockets that can pass the electrical performance requirements can be limited by cost or the amount of use before they must be repaired or replaced. It can take hours or days to replace the failing socket, with the corresponding unit volume of IC devices which could have been tested being a major factor in the overall cost of use (e.g., an opportunity cost). Damage to the PCB used for the test interface can also dramatically impact the cost of use as the PCB's can be very expensive and have a long lead-time to build (and replace).